WASHINGTON, Nov 5 (Reuters) - U.S. Federal Communications Commission Chairman Ajit Pai on Monday wrote the chief executives of major telephone service providers and other companies, demanding they launch a system no later than 2019 to combat billions of "robocalls" and other nuisance calls received monthly by American consumers.

In May, Pai called on companies to adopt an industry-developed "call authentication system" or standard for the cryptographic signing of telephone calls aimed at ending the use of illegitimate spoofed numbers from the telephone system. Monday's letters seek answers by Nov. 19 on the status of those efforts.

Pai's letters raised concerns about some companies current efforts including Sprint, CenturyLink, Charter, Vonage, Telephone and Data Systems Inc and its U.S. Celullar Corp unit and Frontier Communications Corp. The letters to those firms said they do "not yet have concrete plans to implement a robust call authentication framework," citing FCC staff.

The authentication framework "digitally validates the handoff of phone calls passing through the complex web of networks, allowing the phone company of the consumer receiving the call to verify that a call is from the person supposedly making it," the FCC said.

YouMail, a company that blocks robocalls and tracks them, estimated there were 5.1 billion unwanted calls last month, up from 3.4 billon in April.

The FCC has taken a number of actions to try to deter robocalls or automated, prerecorded calls that regulators have labeled a “scourge."

"We need call authentication to become a reality — it’s the best way to ensure that consumers can answer their phones with confidence. By this time next year, I expect that consumers will begin to see this on their phones,” Pai said.

"I am calling on those falling behind to catch up ... If it does not appear that this system is on track to get up and running next year, then we will take action to make sure that it does."

FCC Commissioner Jessica Rosenworcel, a Democrat, earlier this year called on the FCC to set a deadline and noted "Canada went ahead and set a 2019 deadline to put this technology in place. We should be doing the same as our neighbors to the north." (Reporting by David Shepardson; editing by Dan Grebler and Tom Brown)